1. Field of the Invention
The present invention relates to an optical transmission system, and more particularly to an optical transmission system for performing communication using a plurality of communication lines.
2. Description of the Background Art
As an optical transmission system for performing communication using a plurality of communication lines, there is a known technology as follows. FIG. 13 is a block diagram illustrating a configuration of a conventional optical transmission system. In FIG. 13, the conventional optical transmission system includes a multiplexing section 81, an optical modulating section 82, an optical transmission path 83, an optical detecting section 84, a demultiplexing section 85, first to n'th basic modulating sections 86-1 to 86-n, first to n'th electric or wireless transmission paths 87-1 to 87-n, and first to n'th demodulating sections 88-1 to 88-n. An operation of the conventional optical transmission system illustrated in FIG. 13 will be described below.
The multiplexing section 81 multiplexes a plurality of incoming digital data signals. The optical modulating section 82 converts a multiplexed signal obtained by the multiplexing section 81 into an optical signal, and transmits the optical signal over the optical transmission path 83. The optical detecting section 84 reconverts the optical signal transmitted via the optical transmission path 83 into the multiplexed signal. The demultiplexing section 85 demultiplexes the multiplexed signal, which is obtained by reconversion via the optical detecting section 84, into a plurality of digital data signals. The first to n'th basic modulating sections 86-1 to 86-n convert the plurality of digital data signals, which are obtained by demultiplexing via the demultiplexing section 85, into prescribed modulated signals, and then transmit the prescribed modulated signals over the first to n'th electric transmission paths 87-1 to 87-n. The first to n'th demodulating sections 88-1 to 88-n reconvert the modulated signals transmitted via the first to n'th electric or wireless transmission paths 87-1 to 87-n back into the plurality of original digital data signals.
In general, the conventional optical transmission system as illustrated in FIG. 13 is applicable to a digital subscriber line (DSL) service. In the DSL service, an optical transmitting facility 801 including the optical modulating section 82 is provided in a central office (CO) of a telephone service provider or the like. An optical receiving device 802, which includes the optical detecting section 84 and the first to n'th basic modulating sections 86-1 to 86-n, is provided in an elevated position of a telegraph pole, on a sidewall of a subscriber's house, or in a common facility of a multi-dwelling unit (MDU) or a multi-tenant unit (MTU). Each of first to n'th subscriber terminals 803-1 to 803-n including their respective first to n'th demodulating sections 88-1 to 88-n is provided in a subscriber's house or a plurality of subscribers' houses. The “subscriber terminal” as described herein is so called the “customer premise equipment (CPE) device”. For each of the electric or wireless transmission paths 87-1 to 87-n, a subscriber line is used.
In a conventional optical transmission system as described above, it is possible to improve a transmission characteristic of transmitting information across a transmission path from a central office facility to a subscriber terminal by using low-loss optical fibers to form almost the entire transmission path and by transmitting digital signals through that section. Thus, the transmission path has improved performance in transmission quality, transmission rate, etc. It is also possible to simplify the subscriber's in-house network wiring and thereby to reduce the network wiring's cost by using electric lines, such as twisted-pair lines. The subscriber's electric lines thereby form an end portion (from an optical receiving device to the subscriber terminal) of the entire transmission path, through which the DSL modulated signals are transmitted. In this manner, according to the conventional optical transmission system, it is simultaneously possible to lengthen the distance of the entire transmission system and to facilitate installation and cost-effectiveness of the subscriber's in-house facility.
However, the conventional optical transmission system as described above has a problem in that when there is an increase in the number of subscribers, an optical receiving device connected to subscriber terminals is required to be large-scale, resulting in cost increase of the optical receiving device. More specifically, in the configuration illustrated in FIG. 13, the optical receiving device 802 is required to include the first to n'th basic modulating sections 86-1 to 86-n in accordance with the number of subscribers accommodated in the optical transmission system, and therefore the optical receiving device 802 is required to be upsized, thereby increasing the cost of the optical receiving device 802. Since the optical receiving device 802 located in the vicinity of the subscriber is upsized and therefore the cost thereof is increased, the cost effectiveness of the entire optical transmission system is significantly worsened.
In order to solve the above problem, it is conceivable that conversion from a digital signal to a modulated signal is performed in an optical transmitting facility, for example. Specifically, the optical transmission facility generates a plurality of modulated signals corresponding to all the subscriber lines, and sends the modulated signals over an optical transmission path. In this case, the optical receiving device does not require a basic modulating section, and therefore it is possible to solve problems concerning upsizing and cost increase of the optical receiving device.
In the case where the optical transmitting facility performs modulation, when modulation parameters for use in modulation are equally set for all the digital data signals to be transmitted to all the subscriber terminals, a fixed data rate is set for data to be transmitted to all the subscriber terminals. In this manner, when a fixed data rate is set regardless of a communication status of each subscriber terminal, the optical transmission path cannot be efficiently utilized. For example, consider a case where a great deal of data is transmitted to a subscriber terminal A and no data is transmitted to another subscriber terminal B. In this case, even if the optical transmission path has room for transmitting larger amount of data, it is not possible to increase the data rate for the subscriber terminal A. That is, it is not possible to efficiently utilize the optical transmission path.